(a) Field of the Invention
The present invention relates to a thermosetting resin composition for a high speed transmission circuit board, more particularly to a thermosetting resin composition having superior dielectric characteristics with low dielectric constant and dissipation factor and having superior glass transition temperature, heat resistance after moisture absorption, dielectric reliability, adhesion to copper film, workability, dispersibility of inorganic filler, electric characteristics, etc., and thus being useful for a copper clad laminate for high speed signal transfer.
(b) Description of the Related Art
Recently, electronic devices such as computers and communication devices, etc. are being more highly integrated and multilayered. And, mobile phones, notebook computers, etc. are becoming more small-sized, light and capable. As printed circuit boards mounted on these devices are becoming more integrated and compact, superior heat resistance and dielectric reliability are required. With the development of high speed communication services, signal delay and transmission loss of high speed transmission circuit boards mounted on communication devices are emerging as problem. Because the signal delay is proportional to the square root of the dielectric constant of an insulator and the transmission loss is proportional to the dielectric constant of the insulator and the dissipation factor, a material having low dielectric constant and dissipation factor is required for a high speed transmission circuit board.
In the conventional FR-4 printed circuit board, an epoxy resin composition comprising a brominated difunctional epoxy resin, a multifunctional epoxy resin, an amine based curing agent, an imidazole curing accelerator, etc. is used. The polar groups resultant from the reaction of the epoxy resin and the amine based curing agent increase the dielectric constant and the dissipation factor, so that it is impossible to obtain transmission characteristics sufficient for high speed transmission.
In order to solve this problem, Korea Laid-Open Publication No. 2002-0044342 discloses a method of using an acid anhydride based epoxy curing agent and Korea Laid-Open Publication No. 2002-0085475 discloses a method of using a dicyclopentadiene based epoxy resin, which has low dielectric constant. However, epoxy resin is limited in offering sufficiently low dielectric constant and dissipation factor required for high speed and high frequency transmission.
Thus, use of a thermoplastic resin such as a fluorine based resin, polyphenylene ether, etc. or a composite resin of polyphenylene ether and epoxy resin, which have low dielectric constant and dissipation factor, has been proposed. However, these resins require high temperature and pressure for processing because of high melting point or softening point and have inferior processing characteristics and adhesion to copper film.
Use of a cyanate ester resin, which has the lowest dielectric constant among thermosetting resins, offers low dielectric constant and dissipation factor. But, the dielectric reliability worsens because triazine, which is included in the resin, has poor heat resistance after moisture absorption.
In order to solve this problem, Japanese Patent Publication No. 2000-239496 discloses a method of mixing a cyanate ester resin with a dicyclopentadiene based epoxy resin, which has low dielectric constant and dissipation factor, to reduce dielectric constant and dissipation factor and improve heat resistance after moisture absorption. However, this method does not offer electric characteristics sufficient for a high speed transmission circuit board.
In order to improve dielectric characteristics and heat resistance after moisture absorption of cyanate ester, U.S. Pat. Nos. 6,162,876, 6,245,841, Japanese Laid-Open Publication No. 1999-21507 and Japanese Laid-Open Publication No. 1999-21453 disclose a method of decreasing dielectric constant and dissipation factor by mixing a phenol-modified cyanate ester oligomer with polyphenylene ether. However, this method is also limited in that the heat resistance after moisture absorption worsens. Japanese Laid-Open Publication No. 1999-263940 discloses a method of mixing a phenol-modified cyanate ester oligomer with a cyclic olefin copolymer and Japanese Laid-Open Publication No. 2001-214053 discloses a method of mixing a phenol-modified cyanate ester oligomer with polystyrene. However, these methods are limited in that the heat resistance after moisture absorption and the adhesion to copper film is very low because of insufficient compatibility with cyanate ester.
Japanese Laid-Open Publication No. Sho 63-33505 discloses a method of adding an inorganic filler to improve the performance of a clad laminate. However, as the inorganic filler is mixed with varnish, the filler is slowly deposited in an impregnation tank during prepreg preparation, and thereby worsens workability. Also, the appearance of the prepreg becomes poor, the dispersibility of the inorganic filler becomes non-uniform, so that the interfacial adhesion of the clad laminate, drilling characteristics and insulating characteristics worsen.
Japanese Laid-Open Publication No. 2001-339130 discloses a method of improving dispersibility of the inorganic filler, offering the clad laminate formability and processing characteristics comparable to those of an epoxy resin and improving heat resistance and dielectric reliability by adding an inorganic filler treated with a silicon polymer having an hydroxyl group to a cyanate ester resin and polyphenylene ether. However, this method requires an additional process of treating the inorganic filler with the silicon polymer. And, the content of the inorganic filler should be increased to increase heat resistance after moisture absorption of the clad laminate, which increases the viscosity of varnish and thus makes the processing difficult.
Accordingly, a thermosetting resin having superior dielectric characteristics with low dielectric constant and dissipation factor and being capable of improving heat resistance after moisture absorption and dispersibility of inorganic filler is necessary.